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NEW YORK (CNN) -- Ali Saleh Kahlah al-Marri, a 40-year-old from Qatar, is the only enemy combatant held in the United States whose case remains in legal limbo.

This week marks the alleged al Qaeda operative's fourth year in U.S. custody.

Since his arrest on credit card fraud charges in December 2001, al-Marri has remained in solitary confinement, his attorneys said. They are suing the government to improve his jail conditions and to challenge the constitutionality of his detention.

Since the September 11, 2001, attacks, the government has detained three men as enemy combatants. One, Jose Padilla, recently was indicted. A second, Yaser Hamdi, has been deported. And then there is al-Marri.

Mohammed al-Marri, the prisoner's 47-year-old brother in Qatar, said in a statement to CNN that the family wants him to have "his fair day in court."

Wants his day in court

"He has been taken away from his family and denied justice for four years," the brother said. "If he's done something wrong, then the United States must prove it in a court of law. Otherwise, they should let him go so he can return to his family and country."

Al-Marri arrived in the United States the day before the September 11 attacks. He was a computer science graduate student at Bradley University in Peoria, Illinois, where he had earned an undergraduate degree a decade earlier.

By mid-December, the government claims, al-Marri had missed so many classes he was on the verge of flunking out.

Originally, al-Marri's arrest in Peoria, where he lived with his wife, two sons and three daughters, didn't appear to be connected to international terrorism. Criminal charges alleged he possessed a handwritten list of three dozen credit card numbers belonging to others, and more than 1,000 additional card numbers in his laptop computer files.

But at an early court hearing, a prosecutor said al-Marri was believed to be an associate of al Qaeda, the Islamic terrorist group that carried out the September 11 attacks.

The case against al-Marri escalated as investigators more closely inspected his computer and interrogated al Qaeda detainees, the government claims.

Computer files searched

Eventually, evidence was disclosed indicating that a telephone card belonging to al-Marri had been used to call a number in Dubai linked to Mustafa al-Hawsawi, a reputed al Qaeda financier who had wired money to some of the September 11 hijackers and admitted al Qaeda conspirator Zacarias Moussaoui.

Al-Hawsawi was captured in Pakistan in March 2003 and is detained in an undisclosed location outside the United States.

Al-Marri's computer also stored Arabic lectures by al Qaeda leader Osama bin Laden, photographs of the September 11 attacks, and a cartoon of planes crashing into the World Trade Center, according to the government.

It had a folder labeled "jihad arena" and another called "chem" containing industrial chemical distributor Web sites from which, according to the government, al-Marri got information about hydrogen cyanide, a poisonous gas used in chemical weapons.

Al-Hawsawi is believed to be one of the "multiple sources" cited by the Pentagon to support President Bush's June 2003 declaration naming al-Marri an "enemy combatant" and transferring him to military custody.

The declaration alleges that al-Marri engaged in "hostile and warlike acts" working as an "al Qaeda sleeper agent" who was planning to "hack into the computer systems of U.S banks," and possibly facilitate a follow up to the September 11 attacks.

The Pentagon asserts al-Marri trained at a terror camp in Afghanistan before September 11, met bin Laden, and volunteered for a "martyr mission."

Allegations denied

"It's not true, and it's never been established by any evidence that's been provided certainly to us or in a court of law," said New Jersey-based defense attorney Mark Berman. "What we're trying to obtain on his behalf is an opportunity to confront whatever evidence the government has so we can, in fact, establish that he is not what the president says he is."

After al-Marri was sent to the U.S. Navy brig in Charleston, South Carolina, the military refused to allow his attorneys to visit him for 16 months.

When they finally did, they heard tales from their client of being deprived of shoes and socks, blankets, toilet paper, toothpaste and sunlight.

Al-Marri complained that he was subjected to constant video surveillance, repeated interrogations and threats against his family overseas.

But worst of all, he said, was the isolation.

"Mr. al-Marri has been detained at a naval brig for two-and-a-half years in cell that is 9 feet by 6 feet." said Jonathan Hafetz, of the Brennan Center for Justice at New York University Law School.

"During that time he has long been denied books, news, any contact with the outside world other than his attorneys, including his wife and five children, who he has neither seen nor spoken to," he added. "I mean things that we don't even do to people who've been convicted of crimes."

The military has denied in court papers that al-Marri was mistreated.

Al-Marri's two lawsuits, challenging his treatment and the legality of his detention, are pending in U.S. District Court in South Carolina.